Correctly regulated epidermal proliferation and differentiation are vital for skin homeostasis and are disrupted in many human skin diseases, including psoriasis, cancer, and chronic wounds. Understanding the control of epidermal growth and differentiation at the level of gene regulation has been the longstanding thematic focus of AR45192 and remains so in this revised competing renewal. The current application focuses on histone-modifying epigenetic gene regulatory proteins of the Polycomb group (PcG) as well as on the PcG-antagonizing histone demethylase, JMJD3. First, we will extend our recent findings indicating that PcG chromatin-modifying epigenetic repressors of gene expression are required to suppress entry into the epidermal differentiation pathway. We will define the role of specific PcG proteins and PcG complexes in this process to gain greater mechanistic insight into PcG regulation of epidermal homeostasis. Functional studies of members of the 3 major PcG complexes, PRC1, PRC2 and PhoRC will determine if PcG DNA binding components help target PcG complexes to differentiation genes, will examine the conventional model in which PRC2 effects are contingent upon PRC1 and will define the functional overlap of PRC2 histone methyltransferase paralogs in controlling epidermal differentiation. These studies are designed to further characterize the action of specific PcG epigenetic regulators in epidermal growth and differentiation. Second, we will characterize the function of the recently identified PcG-antagonizing histone demethylase, JMJD3, in early progression towards epidermal neoplasia. We recently observed that JMJD3 is required for epidermal differentiation. In normal epidermis, JMJD3 resides within the nucleus, where it binds differentiation gene promoters, however, JMJD3 is mis-localized to the cytoplasm in undifferentiated epidermal cancers. We also observed that suppression of epidermal differentiation by the Ras oncogene, implicated strongly in epidermal neoplasia, is functionally opposed by JMJD3, suggesting that loss of JMJD3 nuclear function may be important in the impaired differentiation that accompanies carcinogenesis. We will therefore test the role of JMJD3, and its subcellular localization, in epidermis undergoing neoplastic transformation. These studies are designed to define the role of the JMJD3 histone demethylase in epidermal neoplastic progression At the end of the proposed support period, we hope to have characterized the mechanistic actions of both PcG and JMJD3 gene regulatory proteins in the control of epidermal proliferation and differentiation as a foundation for the future development of new treatment strategies for human disorders of characterized by disrupted epidermal homeostasis.